Introduction. Survival of an organism requires that both the osmotic concentration and the volume of the extracellular fluid (ECF) be maintained constant. Since Na and its attendant anions are the major osmotic constituents of the ECF, the organism must control the amount of salt as well as the amount of water in this fluid. This is done primarily by controlling the rate of excretion of these substances; control of the intake of salt and water is usually but not always, of secondary importance. Within the kidney, the rate of excretion of salt and water is controlled by regulation of GFR and by regulation of the rate of tubular reabsorption. To a great extent, water reabsorption occurs as a consequence of active salt reabsorption. Therefore, the primary mechanisms involved in the maintenance of the osmotic concentration and volume of the ECF are the active transport systems for salt.

A. The proximal tubular epithelium has a high permeability (L_pA) to water and a high conductance (g) for small ions. The active transport mechanisms are high rate-low gradient processes. (Table 3-1).

B. 60 to 70% of the filtered water and solute is reabsorbed with little change in the osmotic concentration or Na concentration of the tubular fluid. The HCO₃ concentration drops to a moderate extent in the initial 20% of the tubule and then remains constant for the remainder of the length of the tubule. The Cl concentration rises somewhat in that initial 20% and then also remains constant (Fig. 6-1).

Fig. 6-1. The transepithelial chemical and electrical gradients along the length of the proximal tubule. TF/P = tubular fluid to plasma concentration ratio; V_t = transepithelial voltage gradient; A.A. = amino acids.

C. In the initial 20% of the proximal tubule, the lumen is slightly negative with respect to the ISF. The remainder of the proximal tubule is slightly positive. Although the electrical gradient is small, the very high conductance makes it possible for that small force to drive large fluxes.

D. The primary mechanism driving all transport in the proximal tubule is the Na-K ATPase mechanism located only in the basolateral membrane of the tubular cells (Fig. 6-2). In transporting Na out of the cell into the paracellular space and K into the cell, it reduces the cell Na concentration to a low level and raises the cell K concentration. The concentration gradient and the presence of a significant K conductance renders the cell electrically negative with respect to its surroundings. In the steady state the pump operates far below the saturation point for Na and thus an increase in Na entry into the cell across the apical membrane increases the pump rate.

E. Na entry into the cell from the lumen occurs via a number of mechanisms all driven by the chemical or electrochemical gradient for Na created by the Na-K pump in the basolateral membrane (Fig. 6-2).

1. A Na-H antiport is present in the apical membrane (Fig 6-2). This electrically neutral 1:1 transporter is driven primarily by the chemical gradient for Na across the membrane. This is a major mechanism for Na entry.

2. A Na-glucose symport is also present in the apical membrane (Figs. 5-3, 6-2). This transporter is driven by the electrochemical gradient for Na and transports net charge into the cell. This transporter is present all along the tubule but ordinarily the glucose concentration drops to virtually zero in the initial segment (Fig. 6-1), Na-amino acid cotransporters are also present.

3. A small number of Na channels are thought to exist in the apical membrane. Thus, Na can enter the cell by passive diffusion down the electrochemical gradient. This is a minor mechanism for Na entry.

Fig. 6-2. The mechanisms for Na reabsorption in the proximal tubule.

F. The rate of Na reabsorption in the proximal tubule is severely gradient-limited. The high conductance of the tight junction to Na permits a high rate of passive diffusion (Fig. 6-2). If the concentration of Na in tubular fluid begins to fall as a result of the active reabsorption, the rate of passive diffusion down the chemical gradient into the tubular lumen via the paracellular pathway quickly rises. Therefore the net rate of Na reabsorption is high only if water is reabsorbed at a high rate so that the Na concentration is prevented from falling. Usually water is reabsorbed at a high rate and thus a high rate of Na reabsorption normally occurs. This is a high rate, low gradient mechanism.

G. The bicarbonate anion is reabsorbed as a result of proton secretion by the Na-H antiport in the apical membrane (Fig.6-3).

1. The secreted proton reacts with filtered HCO₃ to form H₂O and CO₂. The CO₂ then diffuses out of the lumen.

Fig. 6-3. The mechanism for HCO₃ reabsorption in the proximal tubule.

2. Within the cell the loss of the secreted proton drives the reaction of H₂O and CO₂ in the opposite direction to produce a proton and a bicarbonate anion.

3. The HCO₃ ion is transported out of the cell across the basolateral membrane by a HCO₃-Na symport in a ratio of 3:1. This transport is driven primarily by the electrical gradient for HCO₃.

4. The whole process causes the disappearance of a HCO₃ anion from the tubular fluid and the transport of another HCO₃ anion into the interstitium, the same result as would occur if HCO₃ was transported as such across the cell layer. Thus, the process is called HCO₃ reabsorption.

5. The enzyme, carbonic anhydrase, speeds the rate of HCO₃ reabsorption. It is attached to the luminal surface of the brush border and is in contact with the luminal fluid. It catalyzes the dehydration of H₂CO₃ in the tubular fluid. The enzyme is also present within the cell and there it catalyzes the hydration of CO₂. Both reactions speed the rate of bicarbonate reabsorption.

6. Two gradients affect bicarbonate reabsorption. First the rise in proton concentration in the tubular fluid (fall in pH) tends to counter the Na gradient driving the Na-H antiporter. Secondly, the HCO₃ gradient across the epithelium causes some passive back-diffusion of that ion.

7. HCO₃ is reabsorbed at a faster rate than Cl in the initial segment of the tubule so its concentration falls and that of Cl rises (Fig. 6-1). In the later segments of the tubule, reabsorption continues at a steady pace as water is reabsorbed and the bicarbonate concentration remains constant.

8. Na-H exchange is stimulated by angiotensin II and inhibited by carbonic anhydrase inhibitors such as acetazolamide.

H. Cl is reabsorbed actively via a cellular path and passively via the paracellular pathway. Fig. 6-4 illustrates both pathways.

1. A Cl-base exchanger exists in the apical membrane and operates in a coupled manner with the Na-H exchanger. The base anion, that is transported into the lumen in exchange for Cl, is titrated by secreted protons. The uncharged product Hbase then diffuses into the cell where it dissociates and the products are recycled by the paired exchangers. The base may be OH^-, formate (HCO₂^-), oxalate, or HCO₃. The net result of the coupled action of the two exchangers is the transport of Na and Cl into the cell. Cl exits the cell across the basolateral membrane via a K-Cl symport mechanism and a Cl channel.

Fig. 6-4. Mechanisms for Cl reabsorption in the proximal tubule.

2. The rise in Cl concentration in the early section of the tubule (Fig. 6-1) establishes a chemical gradient for Cl diffusion through the paracellular pathway (Fig 6-4). This causes the V_t in the later sections of the proximal tubule to reverse so that the lumen is positive. The positive V_t in turn drives Na movement through the paracellular pathway.

I. There are segmental differences in the rates of the transport mechanisms for Na and anions within the proximal tubule. In the first third of the proximal tubule, Na is reabsorbed primarily via the Na-H exchanger, and to a lesser extent via the Na-glucose and Na-amino acids symports. This accounts for the fall in the HCO₃ concentration in the tubular fluid and the disappearance of glucose and amino acids.  Cl is reabsorbed to a lesser extent in this segment. In the latter two-thirds of the tubule, Na is reabsorbed primarily with Cl via the coupled Na-H and Cl-base exchangers and via the paracellular path.

J. Water is reabsorbed as a result of a small osmotic gradient created across the proximal tubular epithelium. The transport of all the above solutes from the tubular lumen into the paracellular space tends to drop the osmotic concentration of the tubular fluid slightly and to raise that of the paracellular fluid to a small degree. This establishes an osmotic gradient for water to move from the lumen through the cell into the paracellular space (Fig 6-5). The osmotic gradient is very small (4 to 6 mOsM/kg H₂O), so small that it has not been possible to measure a difference until very recently. Although the magnitude of the gradient is small, the L_pA is quite large and the product of the gradient and L_pA drives a rapid rate of water reabsorption, 72-84 ml/min for the two kidneys. The flow of water into the paracellular space raises the hydrostatic pressure within that space. That pressure drives fluid and transported solutes across the very permeable basement membrane into the ISF.

K. The reabsorbed solutes and water are returned to the circulation from the paracellular spaces and interstitium because of thepressure gradients across the peritubular capillary wall. In the peritubular capillaries surrounding the tubule, the hydrostatic pressure, P_pc, is quite low, and _b is elevated because of the filtration of fluid without protein upstream in the glomerular capillaries (Fig. 2-2). The balance of these two pressures drives absorption of fluid into the capillaries (Fig. 6-5).

Fig. 6-5. Water reabsorption in the proximal tubule.

L. Summary.

1. In the proximal tubule the active reabsorption of Na is the primary event. Cl, HCO₃ and H₂O reabsorption are secondary events resulting from the active transport of Na.

2. There is an extensive interdependence among these transport systems because of the high L_pA and high conductance of the epithelium. The presence of poorly reabsorbed solute in the filtrate creates an osmotic force that can retard water reabsorption and this will inhibit net solute transport by the gradient-limited systems. Conversely, the inhibition of any of the solute reabsorptive mechanisms will inhibit water reabsorption and this will lead to a reduction of net transport by all the low-gradient systems.

3. The rate of salt and water transport by the proximal tubule is sensitive to the magnitude of the hydrostatic and colloid osmotic pressures in the peritubular capillary.

4. All the reabsorptive processes for salt and water in the proximal tubule operate in such a fashion that there is a bulk reabsorption of salt and water from the glomerular filtrate. The rate of return of that reabsorbed fluid to the circulation has little effect on the composition of the circulating plasma but can have a major effect on its volume.

QUESTIONS:  
1. What is the primary transport process in the proximal tubule? Where is metabolic energy utilized in this process? What is the effect of the active transport process on cell electrolyte composition? On ion and electrical gradients across each membrane?

2. What combination of factors causes net Na transport across the cell layer in the reabsorptive direction? What major property of the tubular epithelium limits the net rate of reabsorption?

3. By what types of mechanisms does Na gain entry into the proximal tubular cell across the apical membrane? What drives those mechanisms? At what point is metabolic energy utilized in maintaining the activity of these mechanisms?

4. What transport processes accomplish bicarbonate reabsorption? What is the role of carbonic anhydrase in this process? What limits the net rate of bicarbonate reabsorption in the proximal tubule?

5. What transport processes are involved in chloride reabsorption? What gradients drive these processes?

6. What is considered to be the primary force that begins the process of water reabsorption in the proximal tubule? What additional forces are involved? At what points in the system is metabolic energy utilized? The forces involved in moving water across each cell membrane are quite small; what property of these membranes makes these small forces quite effective?

7. What might be the effect on the rate of water reabsorption of an increase in efferent arteriolar constriction? Of a fall in the filtration fraction?

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